Secretory component (SC) is usually attached to secretory IgA (sIgA). It is made in the secretory epithelial cells that line the mucosal surface. It has been found in this project that the cancers developing from the secretory epithelial cells retain their ability during the malignant transformation to make SC. Thus SC can serve as a marker for these malignant cells, especially when they become metastatic to non-epithelial parts of the body. There is some evidence that the SC made by the tumor cells "spills over" into the bloodstream. If this is so, a measurement of SC in the blood might serve as a screening test for cancer. This possibility is being explored by the development of an assay system for SC, and then by the measurement of SC in the blood of patients with various forms of cancer, and also with other diseases or no disease. Also, SC is being measured in various effusions which might be caused by metastases, to see whether or not these effusions can be differentiated from effusions of infectious or other origin by the presence of SC. Lastly, the basic nature of the tumor SC molecule is being compared with that of normal SC.